Biology 320 Invertebrate Zoology Fall 2005

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Biology 320Invertebrate ZoologyFall 2005

• Chapter 12 Phylum Mollusca
• Part One

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Introduction

• Second largest phylum at 100,000 described spp.
• Chitons, snails, clams, and squids
• Rich fossil record, as calcareous shell preserves
  easily
• Seven classes
• Mostly marine, but some freshwater and
  terrestrial spp.

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Generalized Mollusc

• Also sometimes referred to as HAM (hypothetical
  ancestral mollusc)
• Seven classes share common features
• Amendments to general body plan
• Decent with modification
• General mollusc shape
• Bilaterally symmetrical
• Dorsoventrally compressed
• Oval outline

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• General mollusc structures
• Poorly defined anterior head
• Dorsal visceral mass
• Ventral muscular foot
• Feeding apparatus known as radula
• Mantle
• Shell

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Foot

• Broad, flat, and muscular
• Located ventrally
• Functions in adhesion and locomotion
• Contains secretory mucus glands which facilitate
  locomotion
• Posses several pairs of pedal retractor muscles
• Connect foot to shell
• Contractions of theses muscles allow animal to
  pull shell over visceral mass and foot, or vice
  versa

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Mantle

• Dorsal covering of visceral mass
• Secretes shell
• Forms mantle cavity
• Dorsal pocket that seawater flows through
• Gills are housed here
• Important for many processes such as respiration,
  excretion, and feeding in some cases

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Shell

• Mantle epidermis secretes proteins and calcium
  salts that form the shell
• Three layers to the shell
• Periostracum outermost and proteinaceous
• Ostracum middle and calcareous
• Hypostracum innermost and calcareous may be
  nacreous
• Shell increases in size as animal grows

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Respiration

• Gills are termed ctenidia
• Several pairs in HAM
• One pair or one gill in modern molluscs
• Housed in mantle cavity
• Gills are attached to mantle via axis
• Axis houses branchial blood vessels
• Afferent delivers deoxygenated blood from body
  to gills
• Efferent delivers newly oxygenated blood from
  gills to heart then on to body

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• Leaf-like gill filaments radiate from axis
• Bipectinate if radiate from both sides of axis
• Monopectinate if only one row of gill filaments
• Interfilamentary water spaces separate individual
  gill filaments
• The position of the gills in the mantle cavity
  divides the cavity into inhalant and exhalant
  chambers

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• Gill filaments have cilia that generate water
  currents
• Water enters inhalant chamber ventrally
• Passes between gill filaments
• Exits through the dorsal exhalant chamber
• This system is an example of countercurrent gas
  exchange, which is common in aquatic animals
• Water and blood flow opposite of each other
• Maintains diffusion gradients for O2 and CO2

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Digestive System and Nutrition

• HAM is a browser scrapes algae and small
  organisms off of substratum
• As a result of this feeding strategy, a great
  deal of substrate, minerals, and other inorganic
  particles are ingested
• Digestive system is adapted for browsing
• Ciliary sorting fields to separate food from
  non-food
• Foregut and hindgut are lined with cuticle to
  protect from abrasion
• Gut is less complicated in molluscs that feed on
  larger organic particles

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• Foregut mainly for ingestion
• Mouth
• Buccal cavity
• Radula housed here
• Salivary glands particles are trapped in mucus
  and transported in strings by cilia
• Pharynx

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• Radula
• Feeding apparatus
• Long chitinous structure consisting of many rows
  of curved teeth
• Supported by a connective tissue structure called
  the odontophore
• Protractor and retractor muscles control
  odontophore
• Food is scraped from substrate and pulled towards
  mouth
• Anterior radular teeth are oldest, and one to
  five new rows grow daily

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• Midgut sorting, digestion, and absorption
• Esophagus (considered to be part of foregut in
  Ch. 9)
• Stomach
• Located in visceral mass
• Site of extracellular digestion
• Possesses chitinous gastric shield
• Sorting fields that sort particles by type / size
• Digestive ceca
• Connect to stomach
• Produce enzymes and deliver to stomach
• Site of absorption, intracellular digestion, and
  nutrient storage

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• Hindgut elimination
• Intestine
• Anus
• Located in exhalant chamber
• Wastes are swept away during ventilation

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Coelom

• Coelomate
• Small coelom
• Not used as hydrostat as most have a shell that
  serves as an exoskeleton
• Coelom houses heart, and gonads in HAM
• Protostomes
• Spiral cleavage
• Schizocoely
• Blastopore becomes mouth
• Have not encountered pseudocoelomate animals yet
• We will discuss Phylum Nematoda later

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Hemal System

• Classified as an open system
• Blood is not always contained in vessels
• Hemocoelic tissues and organs are bathed in blood
• Hemal system components
• Heart
• Vessels
• Hemocoel sinuses
• Head
• Foot
• Visceral mass
• Hemolymph containing hemocyanin

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Excretion

• Pair of metanephridia in close proximity to
  pericardial cavity (coelom)
• Hemolymph filtration occurs in visceral sinus of
  hemocoel
• Urine is dumped into exhalant chamber of mantle
  cavity via nephridiopores

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Nervous System

• CNS
• Esophageal nerve ring with pairs of ganglia
• Two pedal and two visceral nerve cords
• Additional pairs of ganglia
• Cerebral ganglia brain that receives sensory
  input from eyes, tentacles, and statocysts
• Buccal ganglia controls odontophore
• Pedal ganglia controls muscular foot
• Pleural ganglia controls mantle
• Sensory organs
• Cephalic tentacles located on head
• Ocelli located on head
• Statocysts located on foot
• Osphradia located in inhalant chamber. Monitor
  incoming water for chemicals and sediment.
  Ciliary beating cessates if conditions are
  unfavorable

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Reproduction

• Gonochoric
• External fertilization
• Gonads attached to coelom (pericardial cavity)
• Gametes released into coelom, enter nephridia,
  and exit nephridiopores into exhalant chamber

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• Most produce a trochophore larva
• Top shaped
• Girdle of cilia called prototroch
• Apical tuft of cilia
• Planktotrophic
• Complete gut
• Some have veliger larvae or direct development

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Class Polyplacophora

• Chitons
• Marine many live intertidally
• Physically challenging habitat
• In some ways resemble HAM
• Shell composed of 8 overlapping plates
• Allows flexibility when conforming to shape of
  substrata

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• Indistinct head lacking eyes and tentacles
• 800 spp.
• 3 mm to 40 cm (gumshoe, Cryptochiton)
• Can be red, brown, yellow, or green in color

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Mantle

• Mantle covers entire dorsal surface, including
  (partially or entirely) the valves
• Thick and stiff
• Has lateral overhangs around mantle cavity
• Referred to as the girdle

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Shell

• Eight overlapping valves
• Name Polyplacophora means bearer of many plates
• Lateral insertion plates of valves are embedded
  in mantle tissue
• Variation in the amount of valve exposed

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• Pair of pedal retractor muscles for each valve
• Four layers to each valve
• Second outermost layer possesses sense organs
  called esthetes (more on this later)

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Locomotion

• Similar to HAM
• Negatively phototactic, so creep away from light
• Found in crevices under ledges and rocks
• If dislodged from substrate (by a wave, predator,
  etc.), chitons can contract their longitudinal
  enrollment muscles, and roll into a ball
• During adhesion, mantle and foot make contact
  with substrate
• Great deal of suction
• Almost impossible to remove animal without
  harming it

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Respiration

• Chitons have two lateral mantle cavities, as
  opposed to HAMs one dorsal cavity
• Located in groove between the foot and mantle
• 6 to 88 bipectinate gills are located on each
  side
• When animal lifts anterior girdle, two inhalant
  apertures form
• Gill cilia beat to draw water posteriorly
• Water exits via one medial exhalant aperture

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Nutrition

• Most are browsers
• Up to 75 of gut contents may be sediment
• Some feed on seaweeds
• Some are carnivorous and may use mantle to trap
  small animals
• Chitons posses very long radulas
• Teeth may be capped with an iron-containing
  mineral called magnetite

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• Digestive system is similar to HAM, with
  exceptions
• Subradular organ
• Located in buccal cavity
• Chemosensory organ that is extended like a tongue
• If food is detected, odontophore is extended
• Two salivary glands
• Also associated with buccal cavity
• Secretes mucus into which particles are trapped
• Strings of mucus with particles are moved to
  esophagus
• Esophagus
• Two esophageal glands dump amylase into esophagus

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Circulation and Excretion

• Hemal system is similar to HAM
• Posterior coelom (pericardial cavity)
• Two huge nephridia in lateral hemocoel

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Nervous System

• Unlike HAM, chitons lack ganglia
• Posses an anterior nerve ring that surrounds the
  anterior gut
• Four longitudinal nerve cords
• Two pedal - ventral
• Two visceral - lateral
• Commissures give nervous system a ladder-like
  appearance

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• Sensory organs
• Subradular organ
• One osphradium in each mantle cavity
• Esthetes
• Unique to chitons
• Found on valves
• Consist of many canals that traverse the layers
  of the valves
• High density of canals up to 1750 / mm2
• Function is disputed

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Reproduction

• Gonochoric
• Single, large, median gonad
• Located in dorsal hemocoel, just anterior to
  coelom
• Two gonoducts that empty directly into exhalant
  chamber
• Gametes do not pass through coelom or nephridia
• Mostly external fertilization, but some internal
  fertilization occurs in mantle cavity of female
• Trochophore larva or direct development